If your group is 100 people or less you might try the Winnipesaukee Belle.
http://www.wolfeboroinn.com/winnipesaukeebelle-en.html
We looked at doing our family reunion aboard the Belle a couple of summers ago and it was very reasonable. If I remember correctly with a sandwich and salad lunch it worked out to about $30 per person for 80-90 people. We ended up just hosting the reunion at our camp because it allowed people to arrive and leave at different times as opposed to the set time of a boat cruise, but the event staff at the Wolfeboro Inn was very easy to work with for quotes, etc.